LeetCode

题目：

Given a collection of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

Each number in C may only be used once in the combination.

Note:

• All numbers (including target) will be positive integers.
• Elements in a combination (a₁, a₂, … , a_k) must be in non-descending order. (ie, a₁ ≤ a₂ ≤ … ≤ a_k).
• The solution set must not contain duplicate combinations.

For example, given candidate set 10,1,2,7,6,1,5 and target 8,
A solution set is:
[1, 7]
[1, 2, 5]
[2, 6]
[1, 1, 6]

思路：

每个数字只能出现一次了，所以递归时直接定位到下一个元素。

package sum;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class CombinationSumII {

    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
        Arrays.sort(candidates);
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        List<Integer> record = new ArrayList<Integer>();
        combine(candidates, 0, candidates.length, res, record, target);
        return res;
    }
    
    private void combine(int[] nums, int start, int end, List<List<Integer>> res, List<Integer> record, int target) {
        for (int i = start; i < end; ++i) {
            List<Integer> newRecord = new ArrayList<Integer>(record);
            newRecord.add(nums[i]);
            int sum = sum(newRecord);
            int rem = target - sum;
            if (rem == 0) {
                res.add(newRecord);
            } else if (rem > 0 && rem >= nums[i]) {
                combine(nums, i + 1, end, res, newRecord, target);
            } else if (rem < 0) {
                break;
            }
            
            while (i + 1 < end && nums[i + 1] == nums[i]) ++i;
        }
    }
    
    private int sum(List<Integer> record) {
        int sum = 0;
        for (int i : record)
            sum += i;
        return sum;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int[] nums = { 10,1,2,7,6,1,5 };
        CombinationSumII c = new CombinationSumII();
        List<List<Integer>> res = c.combinationSum2(nums, 8);
        for (List<Integer> l : res) {
            for (int i : l)
                System.out.print(i + "	");
            System.out.println();
        }
    }

}